1. Field of the Invention
The present invention relates to Orthogonal Frequency Division Multiple Access (OFDMA) wireless communication, and more specifically, to a time synchronization method for Worldwide Interoperability for Microwave Access (WiMAX) system.
2. Description of the Related Art
Orthogonal Frequency Division Multiplexing (OFDM) is a promising technology that is widely used in wireless communication systems, and converts a frequency selective channel to a collection of frequency-flat subchannels achieved by splitting the input high-rate data stream into a number of low-rate substreams. In order to enhance the efficiency of the frequency allocation, the technique of Orthogonal Frequency Division Multiple Access (OFDMA) is proposed to allow multiple users to access a same channel, by dividing available subcarriers into mutually exclusive clusters assigned to distinct users for simultaneous transmission. The orthogonality of the subcarriers guarantees intrinsic protection against multiple access interference, while the adoption of a dynamic subcarrier assignment strategy provides the system with high flexibility in resource management. OFDMA has become a part of IEEE 802.16 standards for wireless metropolitan area networks as a promising candidate for next generation broadband wireless networks.
With more fine frequency allocation, OFDMA signal demodulation is extremely sensitive to timing errors and carrier frequency offsets. Inaccurate compensation of the frequency offset destroys the orthogonality among subcarriers and produces inter-carrier interference (ICI). Timing errors result in inter-symbol interference (ISI) and produce severe error rate degradations.
OFDMA uses a cyclic extension between adjacent symbols to provide intrinsic protection against the time dispersion and timing offset. In the WiMAX standard, IEEE 802.16e, the cyclic prefix (CP) is adopted. As soon as the symbol timing is detected within the cyclic extension, the received spectrum will keep the constant magnitude and only lead to a phase rotation. But if the symbol timing is detected out of the cyclic extension, both the magnitude and the phase of the current symbol are interfered with by the adjacent symbols, which leads to unrecoverable errors in a received frequency spectrum.
In an OFDMA downlink process, the OFDMA symbols are generated after an Inverse Fast Fourier Transform (IFFT) and converted from the tones in frequency domain to the signal in time domain in a base station (BS), while in a mobile station (MS) or subscriber station (SS) the symbols should be detected and converted to the tones in the frequency domain after a FFT unit. The preamble is the first symbol modulated with the predetermined pseudo random sequence and highest power in a time division duplex (TDD) frame. Hence, it is a good signal for detecting the start position of an OFDMA frame and the frequency offset over the channel.
Existing techniques based on preamble detection as time synchronization have some drawbacks.
A popular scheme of preamble detection uses a delay-correlation mechanism, which utilizes the repetitive property of the signal in a time domain. The typical algorithm is CP auto-correlation or M-correlation based on the modulo-3 periodic property in the WiMAX preamble, where M is a number equal to one third of the FFT length. But these correlation results may have a plateau, which affects the position of the symbol boundary and also leads to inaccurate estimation of the fractional frequency offset at that location. In another words, it could lead to an ICI error. Moreover, the M-correlation works badly in multi-cell deployment of a WiMAX system because of the repetition degradation in the preamble. Therefore, the delay-correlation only offers coarse time synchronization under the low signal to noise ratio (SNR) or the time-variant fading channels.
Another technique based on preamble cross-correlation in time domain can offer fine time synchronization, while it is quite sensitive to the carrier frequency offset and should better be processed after the frequency offset compensation. Moreover, this method will produce multi-peaks in the WiMAX preamble because of a modulo-3 property, which makes the peak detection difficult.
Besides the above techniques, the conjugate symmetric property of a BPSK modulated preamble can also be used to perform symbol timing. This method produces several sharp peaks with the biggest peak located at the preamble boundary, but its implementation requires a lot of complex multipliers and delay taps which causes a heavy burden on the system.